A high voltage direct current (DC) power transmission system generates massive harmonic waves in a DC system and an alternating current (AC) system connected therewith, and in order to inhibit output of the harmonic waves at the DC side of a converter, a high voltage passive type DC filter is mounted at the DC side. A common passive filter comprises a mono-tuning filter, a dual-tuning filter, a tri-tuning and a high pass filter. In dual-tuning and tri-tuning circuits, there is a parallel LC (reactor and capacitor) circuit. The protection of the existing dual-tuning and tri-tuning filters is mainly unbalance protection of the capacitor, differential protection of the filters, unbalance protection of two groups of filters having same parameters, impedance protection and voltage ratio protection, and there is no detuning protection method special for the LC parallel circuit therein. Related content may refer to Detection Method Research on Detuning Fault Elements of High Voltage/Ultrahigh High Voltage Tri-Tuning DC Filters (Huihui LUO, South China University of Technology, Master's Thesis, 2011) and DC Filter Protection in Ultrahigh Voltage DC Systems (Jifeng WEN, Power System Automation, Volume 28, Journal 21, Pages 69-72, 2004).
In addition, in respect to a long-distance power transmission mode where a serial capacitance compensator and the like are arranged in a power transmission line, when parameters are inappropriate, a subsynchronous oscillation phenomenon may occur, and the steam turbine-generator shaft may be damaged in serious cases, leading to torsion of the shaft. In order to inhibit the subsynchronous oscillation, one of main methods is to mount blocking filters at the high voltage side of the unit transformer, and the LC parallel circuits are mounted therein. An LC detuning protection method of the blocking filter is mainly the unbalance protection of the capacitor and the out-of-limit protection of an LC current ratio, with respect to a LC current out-of-limit protection method, patent 201310032991.7 discloses a technical solution, but this method, in order to avoid system disturbance or protection maloperation during oscillation, has to expand the upper and lower limit range of a current ratio as well as prolong a protection delay constant value, and these measures reduce the protection sensitivity and cannot provide fast protection.